Do control valves need to shut off?

摘要

On/off valves in use are either kept fully open or fully closed. They are rarely kept partly open. Some designs are not even suitable for use that way. In contrast, control valves are selected and sized to provide a range of flow for a specified pressure drop across the valve. Almost all of them spend their life operating within a minimum and maximum opening to provide the corresponding flows. During normal operation, they are never moved to a fully open or fully closed position.